1. Field of the Invention
The present invention relates to an organic electroluminescence display having two scan driving units for reducing the rising time or falling time of a scan signal and a method of operating the organic electroluminescence display.
2. Discussion of the Background
Organic electroluminescence displays are flat self-emitting displays which emit light by applying an electric field to fluorescent substances coated on a glass substrate or a transparent organic layer. Electroluminescence is a phenomenon whereby fluorescent substances supplied with an electric field emit light.
FIG. 1 shows an energy level diagram for an organic electroluminescence element.
Referring to FIG. 1, an organic electroluminescence element has a structure that an organic thin layer 100 is disposed between an anode, which is a transparent electrode such as ITO (Indium Tin Oxide), and a cathode made of metal having a low work function.
When a forward voltage is applied to the organic electroluminescence element, holes are injected from the anode and electrons are injected from the cathode. The injected holes and electrons couple together to form excitons. The excitons carry out radiative recombination by emitting light during recombination.
The organic electroluminescence element includes a hole injecting layer (HIL) 101, a hole transporting layer (HTL) 103, a light emitting layer (EML) 105, a hole blocking layer (HBL) 107, an electron transporting layer (ETL) 109, and an electron injecting layer (EIL) 111. The organic electroluminescence element is formed in a multi-layered structure because the holes and electrons vary greatly in mobility through an organic material. Since the mobility of electrons is much greater than the mobility of holes, imbalance in density between the holes and the electrons in the light emitting layer 105 occurs. Accordingly, the hole transporting layer 103 and the electron transporting layer 109 are used to effectively transport the holes and the electrons to the light emitting layer 105.
A method of lowering an energy barrier for injecting holes by additively inserting the hole injecting layer 101, made of conductive polymer or copper (Cu) alloy, between the anode and the hole transporting layer 103 can be also used. In addition, by adding a thin hole-blocking layer 107 made of, for example, Lithium Fluoride (LiF) between the cathode and the electron transporting layer 109, the energy barrier for injecting electrons can be reduced to enhance the light emission efficiency, thereby reducing the driving voltage.
The organic electroluminescence display is classified into a passive matrix type and an active matrix type, depending upon the driving methods.
The passive matrix electroluminescence display is a device where anodes and cathodes extend perpendicularly to each other and are disposed to intersect each other in a matrix shape. Pixels are formed in the intersections between the anodes and the cathodes.
Conversely, the active matrix electroluminescence display is a device where a thin film transistor is formed in each pixel and each pixel is individually controlled by using the thin film transistor (TFT).
The emission times for active matrix type and passive matrix type organic electroluminescence displays vary greatly. The passive matrix electroluminescence display allows an organic light-emitting layer to instantaneously emit light with high brightness, but the active matrix electroluminescence display allows the organic light-emitting layer to continuously emit light with low brightness.
With the passive matrix type, the instantaneous emission brightness is increased in order to increase resolution. In addition, since it emits light with high brightness, the organic electroluminescence display easily deteriorates. On the contrary, in case of the active matrix type, since the pixels are driven using the TFTs and continuously emit light for one frame, they can be driven with low current. Therefore, the active matrix type has parasitic capacitance and power consumption lower than those of the passive matrix type.
However, the active matrix type has a defect: brightness is not uniform across the panel. The active matrix type mainly employs a Low Temperature Poly Silicon (LTPS) TFT as an active element. The LTPS TFT is comprised of crystallized amorphous silicon, which is formed in a low temperature by using a laser. However, the characteristics of each thin film transistors can vary due to variations in crystallization. Specifically, threshold voltages of the transistors are not uniform pixel by pixel. Therefore, individual pixels can exhibit different brightness levels with the same image signal, which causes non-uniform brightness difference across the panel
The problem of non-uniform brightness may be solved by compensating for the characteristics of driving transistors. Compensation for the characteristics of the driving transistors is classified into two kinds according to driving type: voltage programming method and current programming method.
The voltage programming method is a technique for storing the threshold voltages of the driving transistors in capacitors and compensating for the stored threshold voltages of the driving transistors.
In the current programming method, an image signal is supplied in current and a source-gate voltage of a driving transistor corresponding to the image signal current is stored in a capacitor. Then, the driving transistor is connected to a voltage source and the same current as the image signal current is allowed to flow in the driving transistor. Essentially, the value of current applied to the organic light-emitting layer is a value of the image signal current, regardless of the characteristic difference between the driving transistors. Therefore, the non-uniform brightness is corrected.
Another manner of compensating for brightness, by using a driving circuit, is not a technique for compensating for the characteristics of the driving transistors but a technique for allowing the driving transistors to work in a region having small variation.
FIG. 2A shows a block diagram of a conventional organic electroluminescence display.
Referring to FIG. 2A, the conventional organic electroluminescence display has a scan driving unit 201, a first data driving unit 203, a second data driving unit 205, and a pixel array unit 207 in which pixels are arranged in a matrix shape.
The scan driving unit 201 supplies scan signals to the pixel array unit 207 through scanning lines 1-m (SCAN[1]-SCAN[m]) and supplies control signals to the pixel array unit 207 through emission control lines 1-m (EMI[1]-EMI[m]).
The first data driving unit 203 and the second data driving unit 205 supply data signals to pixels selected by the scan signals from the scan driving unit 201. The data signals are programmed in the pixels selected in a current or voltage type. When the programming operation is finished, the scan driving unit 201 supplies the emission control signals to the selected pixels, thereby allowing the organic electroluminescence elements to emit light.
The pixel array unit 207 includes a plurality of pixels arranged in a matrix shape. Each pixel has an organic electroluminescence element for emitting light and a driving circuit for controlling the emission operation of the pixel. Each pixel is connected to a data line for transmitting a data signal, a scanning line for supplying a scan signal, an emission control line for supplying an emission control signal, and an ELVdd line (not shown) for supplying current necessary for emission of the organic electroluminescence element.
FIG. 2B shows a timing diagram of a conventional organic electroluminescence display.
Referring to FIG. 2A and FIG. 2B, when the scan signal SCAN[1] of the scan driving unit 201 changes from a high level to a low level signal, the pixels of the first row are selected. When the selected pixels are supplied with the data signals from the data driving unit 203 and 205, the selected pixels are programmed. The programming operation of the selected pixels can be carried out in a voltage or current type.
When the programming operation of the pixels of the first row is completed, the emission control signal EMI[1] is supplied to the pixels of the first row from the scan driving unit 201 and the pixels of the first row start emitting light.
The data programming of each subsequent row is carried out sequentially and the programmed pixels sequentially emit light. When the data programming and the emission of the pixels of row [m] are complete, the display of the image signals for one frame is complete.
In the conventional organic electroluminescence display, the scan driving unit is disposed at the left or right side of the pixel array unit and drives a plurality of pixels disposed in a row. When the pixels of the first row are selected, the pixels disposed apart from the scan driving unit 201 are supplied with the delayed scan signals. Thus, when the pixels at the end of the first row are selected, the pixels at the start of the second row are also selected. Data signals must be input simultaneously to opposing ends of the first row and the second row due to the delay of signals.
Scan signals in which the delay time is reflected may be applied, but this solution is not desirable because the delay time depends upon the line resistance of the scanning lines and the capacitance of the pixels. However, since the constants that affect the time delay are slightly different for each pixel, time delay cannot be determined with certainty.